The traditional optical system is fundamentally restricted by the diffraction limit, which is 0.5λ/NA (λ is the wavelength, NA is numerical aperture of the optical system). Breaking the diffraction limit to achieve super-resolution focusing and imaging is of great scientific significance and application value. Far-field super-resolution optical dark spots is attractive for various applications, such as optical microscopy, optical tweezers, nanolithography and ultra-high density data storage. As a common way, the azimuthally polarized wave is used to generate tight focused dark spot with conventional lens. However, high numerical optical lenses are bulky, expensive，and more important, and the conventional optics are diffraction-limited. The super-oscillatory planar lens provides a new technical approach to achieve far-field super-resolution optical dark spots, and their small size and ultra-thin thickness make them promising in system minimization and integration for optical system.

The research group of Professor Chen G, School of Optoelectronics Engineering, Chongqing University, is dedicated to the researches in sub-diffraction optical devices and super-resolution imaging systems. To generate the super-diffraction hollow ring, they proposed a binary-amplitude (0, 1) super-oscillatory planar lens based on concentric ring structure of metal. The working wavelength is 632.8 nm, the lens radius is 650λ, and its focal length is 200λ. The corresponding numerical aperture is 0.96. The lens was designed by the vectorial angular spectrum diffraction theory and the particle swarm optimization. In order to verify the design of the lens, numerical simulation was conducted using COMSOL Multiphysics software, the FWHM of the hollow ring is 0.346λ, and the simulated results are in good agreement with the theoretical design. According to the design results, the concentric ring structure was formed by electron beam lithography and inductively coupled plasma etching, and the device was fabricated. To further study the focusing characteristics of the lens, an optical microscopy system with high numerical aperture was employed to measure the super-oscillatory hollow ring. The experimental results demonstrated the generation of a hollow spot with circular ring shape on the focal plane. The inner full-width-at-half-maximum of the hollow spot is 0.368λ, smaller than the super-oscillatory criterion (0.38λ/NA). Good agreement was found between the experimental and simulated results. Such planar lenses are easy to be fabricated and integrated.

About team

The innovation research group for advanced micro-nano technology is one of the research groups under the Key Laboratory of Optoelectronic Technology and Systems Ministry of Education, Chongqing University. Its research area includes sub-diffraction optical devices and super-resolution imaging system, advanced bio-chemical testing technology and precision medical applications, and micro-nano energy systems. In the area of sub-diffraction optical devices and super-resolution imaging system, there are four major researchers, i.e., Professor Dr. Chen G, Associate Professor Dr. Wen Z Q, Associate Professor Dr. Zhang Z H, and Lecturer Dr. Liang G F. In recent 5 years, with the support of Chinese National 973 Program and Chinese Natural Science Foundation, researches have been conducted in the super-oscillation mechanism, the theories on super-oscillation optical devices, and the corresponding micro-nano fabrication techniques. A variety of super-oscillation focusing lenses have been proposed and demonstrated in visible, near infrared and THz spectrum range for the purposes of super-resolution focusing and imaging. To overcome the limit of short penetration depth in the conventional sub-diffraction focusing devices, our group has proposed a far-field label-free super-resolution microscopy scheme based on sub-diffraction quasi-non-diffraction beam, which can easily penetrate normal experimental sample and achieve a transverse resolution better than 0.33λ. In recent 2 years, the innovation research group for advanced micro-nano technology has published more than 40 academic papers in international journals, and applied for 30 invention patents.